Despite recent doubts, the popular theory that birds evolved from dinosaurs may still be correct.

Researchers have discovered microscopic channels that carried nutrients to bone cells in dinosaurs called coelurosaurs were organised in the same way as they are in birds today.

The find suggests a strong link between this particular group of dinosaurs and modern birds.

The researchers also found evidence that bundles of collagen fibres - which bind bone minerals - have the same irregular structure in both birds and coelurosaurs.

The layers are thicker in some places and much thinner in others, and often they disappear completely before reforming. In modern vertebrates, this type of structure only occurs in bone that forms very rapidly - as it does in birds.

Coelurosaurs were two-legged meat eaters that ranged in size from a chicken to an ostrich.

Theory questioned

The well-established hypothesis that birds evolved from dinosaurs took a severe knock in June.

Longisquama insignis: Fossil feathers predate first birds by 75 million years

A team of Russian and American researchers claimed to have found feathers on the fossil of A small, lizard-like creature that lived 220 million years ago - about the same time that dinosaurs began to roam the Earth - and 75 million years before the first known bird.

Longisquama insignis was an archosaur, part of a group of reptiles from which dinosaurs, birds and crocodiles (birds' closest living relatives) are descended.

The researchers questioned whether feathers could have evolved twice and suggested bird-like creatures may have evolved at the same time as, or even before, the dinosaurs.

Now, the unique links between the bone structure of modern birds and coelurosaurs appear to push the arguments over bird evolution back towards the dinosaurs.

Back on their perch

"Right now, the thing that is closest to what we see in the bones of birds is in the bones of coelurosaurs," Professor John Rensberger, curator of vertebrate paleontology at the Burke Museum, part of the University of Washington, told BBC News Online.

"It doesn't necessarily prove that birds had to derive from dinosaurs. At least from the data we've seen that appears to be a logical conclusion, but there's still plenty of room for argument."

Professor Rensberger and Mahito Watabe, of the Hayashibara Museum of Natural Sciences in Okayama, Japan, made their comparisons using fossilised dinosaur bones collected from the Gobi Desert in China and from the Hell Creek geological formation in Montana.

The researchers sampled about 550 cross sections of bone, ground to a few microns thick, over the course of three years.